WASHINGTON (AP) — In late February when President Donald Trump was urging Americans not to panic over the novel coronavirus, alarms were sounding at a little-known intelligence unit situated on a U.S. Army base an hour's drive north of Washington.

Intelligence, science and medical professionals at the National Center for Medical Intelligence were quietly doing what they have done for decades — monitoring and tracking global health threats that could endanger U.S. troops abroad and Americans at home.

On Feb. 25, the medical intelligence unit raised its warning that the coronavirus would become a pandemic within 30 days from WATCHCON 2 — a probable crisis — to WATCHCON 1 — an imminent one, according to a U.S. official. That was 15 days before the World Health Organization declared the rapidly spreading coronavirus outbreak a global pandemic.

At the time of the warning, few coronavirus infections had been reported in the United States. That same day, Trump, who was in New Delhi, India, tweeted: "The Coronavirus is very much under control in the USA." Soon, however, the coronavirus spread across the world, sickening more than 2 million people with the disease COVID-19 and killing more than 26,000 people in the United States.

The center's work typically is shared with defense and health officials, including the secretary of health and human services. Its Feb. 25 warning, first reported last month by Newsweek, was included in an intelligence briefing provided to the Joint Chiefs of Staff, but it's unknown whether Trump or other White House officials saw it. Various intelligence agencies had been including information about the coronavirus in briefing materials since early January, according to the official, who spoke only on condition of anonymity because he was not authorized to confirm details about the alert.

At least 100 epidemiologists, virologists, chemical engineers, toxicologists, biologists and military medical expert — all schooled in intelligence trade craft — work at the medical intelligence unit, located at Fort Detrick in Frederick, Maryland. Requests to interview current workers were denied, but former employees described how they go through massive amounts of information, looking for clues about global health events.

"You feel like you're looking for needles in a stack of needles," said Denis Kaufman, who worked in the medical intelligence unit from 1990 to 2005 and again later before retiring.

Most of the information they study is public, called "open source" material. A local newspaper in Africa might publish a story about an increasing number of people getting sick, and that raises a flag because there's no mention of any such illness on the other side of the country. A doctor in the Middle East might post concerns about a virus on social media. But unlike organizations such as the WHO, the medical intelligence team, part of the Defense Intelligence Agency, also has access to classified intelligence collected by the 17 U.S. spy agencies.

The medical unit can dig into signals intelligence and intercepts of communications collected by the National Security Agency. It can read information that CIA officers pick up in the field overseas. The National Geospatial-Intelligence Agency can share satellite imagery and terrain maps to help assess how a disease, like Ebola or avian flu, might spread through a population.

"Every day, all of us would come into work and read and research our area for anything that's different — anything that doesn't make sense, whether it's about disease, health care, earthquakes, national disaster — anything that would affect the health of a nation," said Martha "Rainie" Dasche, a specialist on Africa who retired from the DIA in 2018. "We start wondering. We look at things with a jaundice eye."

They don't collect intelligence. They analyze it and produce medical intelligence assessments, forecasts and databases on infectious disease and health risks from natural disasters, toxic materials, bioterrorism as well as certain countries' capacity to handle them. Their reports are written for military commanders, defense health officials and researchers as well as policymakers at the Defense Department, White House and federal agencies, especially the Department of Health and Human Services.

The center was originally in the U.S. Army Surgeon General's office during World War II, but military leaders throughout history have learned the hard way about the danger that disease poses to troops.

"In the Spanish-American War, there were major epidemics of typhoid fever and dysentery," according to a report written in 1951 at Brooke Army Medical Center at Fort Sam Houston in Texas. "World War I saw widespread outbreaks of influenza and malaria. In World War II, there was a high incidence of malaria and infectious hepatitis. ... In all wars prior to World War II, losses from disease exceeded losses from battle injury."

Today, the team's success comes in providing early warnings that prevent illness. That can be difficult if a country doesn't report or share information out of fear that the news will affect its economy or tourism. Some undeveloped countries with poor health systems might not compile good data. Information from countries trying to play down the seriousness of an epidemic can't be trusted.

Kaufman said massive amounts of information come out of China, where the first reports of the new coronavirus surfaced in the city of Wuhan. But because the country is run by an authoritarian government, the medical intelligence researchers glean information from the local level, not Beijing.

"Researchers, in some cases, have more success in learning information from the bottom up — not from the central communist government, but from localities," he said. "That's where some guy in Wuhan might be saying 'I can't report this because I don't want to look bad to my boss' or there's a guy who says he can't talk about avian flu because his cousin runs the bird market and doesn't want to hurt his business."